1. Field of the Invention
The present invention relates to sheet conveying devices for conveying sheets and image forming apparatuses including the sheet conveying devices, such as copy machines, printers, facsimile machines, and compound machines having these functions.
2. Description of the Related Art
Generally, some image forming apparatuses have a construction for image position adjustment shown in FIGS. 6 and 7 in order to adjust the positional relationship between a sheet on which an image is to be formed and the image formed on a photosensitive drum, and the like in an image forming unit.
FIG. 6 is a side view of a sheet-conveying path U along which the sheet is conveyed to the image forming unit of the image forming apparatus. In FIG. 6, reference numeral 104 denotes a photosensitive drum for forming an image, reference numeral 302 denotes a laser for forming a latent image on the photosensitive drum 104, reference numeral 108 denotes resist rollers which determine the time for feeding the sheet such that the positional relationship between the latent image formed on the photosensitive drum 104 and the sheet on which the image is to be formed is adjusted, and reference numeral 304 denotes a sheet-position detection sensor for detecting the front end of the sheet in the sheet-feeding direction.
FIG. 7 is a top view of the sheet-conveying path U. In FIG. 7, P denotes the sheet and reference numeral 2 denotes the sheet-feeding direction.
In operation of the apparatus shown in FIGS. 6 and 7, when the sheet P is conveyed along the sheet-conveying path U, a central processing unit CPU (not shown), which serves as a control unit, determines the position of the sheet P in the sheet-feeding direction 2 by using the sheet-position detection sensor 304 and adjusts the time for transmitting image data to the laser 302 from an image control block (not shown).
However, the above-described operation of the conventional apparatus has the following problem. That is, when the image forming speed of the image forming apparatus increases, the sheet conveying speed also increases, and the sheet-position detection sensor 304 positioned along the sheet-feeding direction 2 must be capable of accurately reading sheets at high speed. However, typical mechanical sheet-position detection sensors used in such devices cause a large error, which directly leads to displacement of the image on the sheet.
In order to solve this problem, an image forming apparatus has been proposed which ensures high resolution and high image position accuracy even when the image forming speed is high by using as the sheet-position detection sensor 304 a sheet-position detection sensor including charge coupled devices (CCDs) or contact image sensors (CISs) in which the reading area can be varied.
However, in the image forming apparatus in which a sheet-position detection sensor including the CCDs or the CISs is used, a detectable area in which a target can be detected (hereinafter referred to as the depth of field) is small, and therefore the distance between the sheet-position detection sensor and the sheet being conveyed must be adjusted precisely. Accordingly, high-precision components are required in order to attach the sheet-position detection sensor with high precision, and fine adjustment must be performed after attaching the sheet-position detection sensor. Therefore, high costs are incurred.
According to the present invention, a sheet conveying device includes a pair of sheet guides forming a sheet-conveying path for conveying a sheet and an image sensor for detecting the sheet as it is conveyed along the sheet-conveying path, wherein the image sensor is retained in a direction perpendicular to the sheet-conveying path, and wherein the image sensor is attached such that a vertical distance from a proximal edge of the image sensor to one of the pair of sheet guides differs from the vertical distance from the distal edge of the image sensor to one of the pair of sheet guides in the direction perpendicular to the sheet-conveying path.
As described above, according to the present invention, the sheet-position detection sensor (image sensor) is attached such that the vertical distance from a proximal edge of the sheet-position detection sensor to one of the pair of sheet guides differs from the vertical distance from the distal edge of the sheet-position detection sensor to one of the pair of sheet guides in a direction perpendicular to the sheet-conveying path. Accordingly, it is not necessary to prepare high-precision components for attaching the sheet-position detection sensor or to reduce a gap between conveyor guide members for guiding the sheet and adjust the sheet-position detection sensor as in the sheet-position detection sensor of the known art. In addition, the depth of field of the sheet-position detection sensor, in which the sheet being conveyed can be detected, is effectively increased, so that it is not necessary to adjust the distance between the sheet-position detection sensor and the sheet being conveyed with high precision. Accordingly, the costs can be greatly reduced and sheet-conveying performance can be greatly improved.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.